1. Field of the Invention
The present invention relates to systems for purging contaminants from the lens of a combustion instrument such as a radiation pyrometer.
2. Description of the Prior Art
Radiation pyrometers are widely used for detecting temperatures in applications such as gas turbines, and the standard pyrometers sense radiation through a lens that is normally mounted in a tube that opens through a housing into a turbine rotor chamber. Accurate determination of turbine blade temperature is necessary in order to enable the engine to be operated at optimal thrust and efficiency, without heating the blades to a point where they warp or lose structural strength. Pyrometry is used extensively as a development tool on research turbines, and is an accepted inflight control/monitor for military aircraft engines. One of the requirements is that the pyrometer lens be kept clean and free of particulate contaminants. Particles deposited on the lens will reduce the energy to the sensor and thereby cause inaccurate temperature measurements. Several purge air designs have been developed to keep particulate contaminants from the lens and the sighting tube, but most have certain limitations in all normal instances, for example when the pyrometers are to be mounted with the sighting tube facing in an upward direction. Gravity will cause particles to fall down onto the lens. In some designs, a particle laden compressor air supply is used to "scrub" the surface of the lens, that is, pass over the surface of the lens. Significant particle deposition on the lens occurs as the result of the contaminants from the purge air flow contacting and adhering to the lens as the air passes over the surface.
Other designs provide an air curtain spaced from the lens that is intended to keep the flow of air in a direction to move the particles away from the lens, but during starting and stopping of the turbine in an aircraft, for example particularly when the pyrometer lens surface is facing upwardly, particles precipitate and accumulate on the lens surface.
A known, simplified, purge air system for a fiber optic cable system is shown in British Patent Specification No. 1,559,185 issued to Smiths Industries Limited. Outward air flow is provided in a housing surrounding the cable end. Air flow in a single passageway past the lens and out into the turbine chamber. Particles in the air are carried across the lens in that no provision is made for removal of such particles.
British Specification No. 2,130,717, owned by Smiths Industries Public Limited Company, shows a more sophisticated purge air system for a pyrometer wherein air flows through a tubular member surrounding the pyrometer lens, which opens directly into a large diameter sight tube, again giving no removal or at best inefficient removal of particles from the air prior to passage of the air over the lens and also provides flow patterns which make it difficult to remove particles which have deposited on the lens.
U.S. Pat. Nos. 4,037,473 and 4,118,985 are based on the same parent application, and show radiation pyrometers that have a purging fluid which is a plenum chamber and provides a purge air flow around the sensing heads but without inertial separation particles from the purge air and the air is not directed across the ends of the sensing heads.
French Pat. No. 77-28335 shows a purge air system substantially similar to that shown in British Specification No. 1,558,185 and used in a radiation pyrometer for turbine blade temperature sensing.
U.S. Pat. No. 4,435,093 shows a pyrometer having a flow of purge air over a lens system, and through diverging passageways illustrated in FIG. 3 of this patent at 40, 41 and 42. The purge air flows through the passageways at least in part across the lens and goes into a large diameter sight tube where it will flow outwardly. The purge air is not subjected to inertial separation of particles before it passes over the lens.
U.S. Pat. No. 4,306,835 shows an air purging unit for an optical pyrometer used in a gas turbine engine that provides angled ports or nozzles spaced downstream from the pyrometer lens, to form a type of a "air curtain" intended to prevent the particles from striking the lens. The problems with ground stops and the like discussed above are present in this device.
U.S. Pat. No. 3,436,965 also shows a purging system for radiation pyrometers, and in this device, it is intended that the air velocity of the purging air would increase as it exits from the sight tube, but, again, the air is to form a curtain that is spaced from the lens in direction toward the sighting opening, and problems that occur when the air velocity is reduced, particularly when the tube is facing upwardly, are present.
Russian Pat. No. 584195 shows a type of shielding device for a pyrometer lens, but without the inertial separation feature of the present invention.